It is known that glasses for the above-mentioned application must be virtually free of alkali metal oxides, since alkali metal ions interfere with the regenerative halogen cycle of the lamp. This is because, during operation of the lamp, the tungsten vapor from the filament and the halogen/inert gas mixture form an equilibrium between formation and decomposition of tungsten halides. The decomposition reaction takes place at higher temperatures than the formation reaction, so that the tungsten is deposited again on the filament. If this cycle is interfered with by contaminating components, such as for example alkali metal ions, the tungsten is deposited not on the filament but on the inside of the glass bulb, as an undesirable shiny black coating. For this reason, high demands are placed on such glasses with regard to their freedom from alkali metal oxides.
Contaminants which have an adverse effect on the regenerative halogen cycle are not only introduced via the glass but also via the actual manufacturing process by which the lamps are produced. The purity of the electrode material used is therefore also of considerable importance. Purification processes, for example pretreatment of the electrodes by annealing, are too complex and expensive. The same also applies to tempering glass bulbs which have a high alkali metal content.
To be usable as lamp bulb glass for lamp bulbs which contain molybdenum components as supply electrodes, the thermal expansion of the glass has to be adapted to that of molybdenum over the entire temperature range of the lamp, in order to ensure a hermetic seal between glass and supply electrode. The glass must have a higher expansion at its setting temperature (setpoint) compared to molybdenum, that is, the difference in expansion between Mo and the glass must be positive, in order to achieve a radial compressive stress in the glass which is beneficial to the sealing of Mo supply electrodes. The glass should therefore have a coefficient of expansion .alpha..sub.20/300 of between 4.4.multidot.10.sup.-6 /K and 4.7.multidot.10.sup.-6 /K.
The glass must also have sufficient thermal stability in order not to deform at the bulb temperatures which occur during operation of the lamp.
A further demand placed on a glass which is suitable for use as bulb glass in lamp bulbs is that it be suitable for tube drawing. For this purpose, the glass must be sufficiently crystallization-stable.
The patent literature already describes glasses for such incandescent lamps.
Like the alkali metal ions, water or hydrogen ions also interfere with the halogen cycle.
For this reason, U.S. Pat. No. 4,163,171 also describes an incandescent lamp in which the glass is not only "essentially free of alkali metals", but also may only contain less than 0.03% by weight of water.
German patent publication 3,305,587 describes incandescent lamps made from glass bulbs which have a high BaO content (11-16% by weight). German patent publication 2,930,249 relates to the use of glass compositions which also have a high BaO content (10-23% by weight) as bulb material. In these compositions, moreover, the BaO:CaO weight ratio is between 2.3 and 3.5 (CaO:BaO between 0.28 and 0.43). The glasses described in the latter document are said to have an improved resistance to what is known as "reboil". Reboil is the tendency of the glass to form a large number of small bubbles of included gases when subjected to further processing with a flame or when reheated. This impairs the light transmission and weakens the reheated areas.
A drawback of glasses, which have very high BaO contents, is that they have high melting and working temperatures by comparison with glasses which have high contents of other alkaline-earth metal oxides. High melting points entail a high power consumption, high melting costs and greater corrosion to the tank furnace material.
U.S. Pat. No. 3,496,401 describes incandescent lamps made from an aluminosilicate glass which contain alkaline earth metals and have a maximum alkali metal oxide content of 0.1% by weight, in particular made from glasses containing SiO.sub.2, Al.sub.2 O.sub.3 and 10-25% by weight of alkaline-earth metal oxides, the specific levels of which are not given in more detail. B.sub.2 O.sub.3 is only an optional component.
U.S. Pat. No. 3,310,413 describes aluminoborosilicate glasses which contain alkaline earth metals for sealing to molybdenum which have defined ratios between the alkaline-earth metal oxides. Thus, with MgO as an optional constituent, the MgO to BaO ratio is limited to less than 0.75 and the ratio of CaO to the sum of MgO and BaO is limited to 0.6 to 2.0.
The glass envelopes for tungsten/halogen lamps described in U.S. Pat. No. 4,394,453 contain large quantities of Al.sub.2 O.sub.3 (at least 16% by weight), which also leads to a relatively low SiO.sub.2 /Al.sub.2 O.sub.3 ratio.
U.S. Pat. No. 4,409,337 also describes glasses for glass envelopes of tungsten/halogen lamps which have low SiO.sub.2 contents relative to the Al.sub.2 O.sub.3 content, expressed by the SiO.sub.2 /Al.sub.2 O.sub.3 weight ratio which in this case is 3.1-3.7.
U.S. Pat. No. 5,489,568 describes glasses which are suitable in particular for use in flat panel displays. These glasses contain either large quantities of Al.sub.2 O.sub.3 (.gtoreq.18% by weight) with relatively low SiO.sub.2 contents (.ltoreq.55% by weight) or little Al.sub.2 O.sub.3 (.ltoreq.13% by weight) with relatively high SiO contents (.gtoreq.55% by weight). The viscosity behavior, which is achieved as a result, is adapted in particular to the technique of drawing flat glass panes and less so to that of tubes for the production of lamps. These glasses are specifically adapted to .alpha.-silicon and polysilicon and less so to Mo. The same applies to the glasses described in European patent publication 0,672,629 and U.S. Pat. No. 5,508,237.
A commercially available glass for the use described is the glass 8252 produced by Schott Glas and having the composition (% by weight rounded) 60 SiO.sub.2 ; 4.5 B.sub.2 O.sub.3 ; 14.5 Al.sub.2 O.sub.3 ; 2 MgO; 10 CaO; 9 BaO. Owing to its profile of properties (for example, .alpha..sub.20/300 4.6.multidot.10.sup.-6 /K, T.sub.g 720.degree. C.), this glass is eminently suitable as a lamp bulb glass for temperatures of approximately 550.degree. C. to 650.degree. C.
However, it is true both for lamps produced from this glass and for lamps produced from the glasses mentioned previously that the function of the lamps is very easily impaired by contaminants, whether they emanate from the glass or from the electrode material of the lamp mount, as evidenced by the above-described black discoloration on the inside of the glass bulb caused by interference with the halogen cycle. It is therefore imperative that very high demands be placed on the purity, and in particular on the freedom from alkali.